1. Field of the Invention
The present invention relates to a method and an apparatus for alkaline saponification of a polymer film, in particular, to a method and apparatus for alkaline saponification of a cellulose ester film, which is used as a transparent support for a continuous optical compensation sheet, in which wastewater used for a water-washing step after alkaline saponification was reused, whereby environmental load as well as costs are decreased.
2. Description of the Related Art
In recent years, there have been increasing demands for optical films. Such optical films include optical compensation sheets, which are used in liquid crystal cells as a retardation plate.
When producing an optical compensation sheet by forming an orientation film and an optically anisotropic layer, in which liquid crystal molecules are fixed, on a transparent support, good adhesion is required between the transparent support (usually, a cellulose ester film represented by a cellulose acetate film) and the orientation film (usually, a polyvinyl alcohol). Since the affinity between a cellulose ester film and a polyvinyl alcohol is weak, stripping or cracking often occurs at the interface thereof; thus, a gelatin undercoating layer has been provided on the cellulose ester film. However, to develop adhesion between the undercoating layer and the cellulose ester film, a solvent that penetrates into the cellulose ester film (e.g. a ketone solvent) must be used as a solvent for the coating fluid for providing a gelatin undercoating layer. The use of such a solvent causes the swelling of a cellulose ester film, which gives rise to a problem of causing fine bending of the film in the course of shrinkage in a subsequent drying step. It was found that if an orientation film and a liquid crystal molecule layer are provided on such a bent film, thickness non-uniformity of the orientation film and liquid crystal molecule layer or orientation non-uniformity of the liquid crystal molecules occurs along the bent shape, resulting in inferior image quality of liquid crystal displays.
On the other hand, as a commonly used method for improving the adhesion between a cellulose ester film and a hydrophilic material (e.g. orientation film) without providing a gelatin undercoating layer, a method is known in which a cellulose ester film is immersed in an alkaline aqueous solution, that is, a so-called saponification bath treatment is conducted. The details of such a saponifying method are described in Japanese Patent Application Laid-Open No. 8-94838. In this saponification bath treatment, in which a cellulose ester film is immersed in an alkaline aqueous solution, both sides of the cellulose ester film undergo hydrophilization simultaneously; thus, a problem occurs, when the cellulose ester film having a hydrophilic layer of polyvinyl alcohol etc. provided on its one side is wound up into a roll, of allowing the two sides of the film to adhere to each other. There is a method, as one example of methods for hydrophilizing one side of the cellulose ester film alone in the saponification bath treatment, in which the cellulose ester film is subjected to saponification treatment with its one side, on which saponification treatment is not intended, made waterproof with a laminate or the like. However, such methods not only increase the number of troublesome steps, but also produce unnecessary wastes, and thus, not being preferable from the viewpoint of productivity and environmental conservation.
In these circumstances, a saponification method was proposed in which a solution of alkali in water or in mixed solvent a organic solvent and water is continuously coated on one side of the polymer film alone on which an orientation film is to be provided, and after the reaction time has elapsed, the alkaline solution is washed away from the polymer film (e.g. Japanese Patent Application Laid-Open No. 2003-313326). According to Japanese Patent Application Laid-Open No. 2003-313326, the alkaline solution is washed away from the polymer film by the method in which washing water is sprayed directly on the polymer film using a spray nozzle or the like through a plurality of water-washing steps.
However, in recent years, because of the increase in production of polymer films and the speed-up of the production process as an evitable consequence of the increase in production, the amount of the washing water used in the water-washing steps have been considerably increased, which presents the problem of raising the production cost.
Further, alkaline saponification involves discharge of a large amount of wastewater containing acid or alkali, which also presents the problem of increasing the load on the wastewater treatment as well as the environmental load.
The present invention has been made in the light of these problems. Accordingly, an object of the present invention is to provide a method and apparatus for alkaline saponification of a polymer film which allow efficient use of washing water and realize low cost and low environmental load while maintaining quality stability.